After he jumped on the thunder pool, he was electrolyte for a moment. It took some time until he adapted to the thunder pool. While being inside the pool, he ran Heavenly Thunder Body Tempering Art. In an instant, a huge amount of thunder energy entered his body.
All that sucked thunder energy made his body feel pain for a long time until he actually used this art to absorb thunder energy inside his cells. The thunder energy was naturally created to improve his physical body so when it was absorbed by his cells, those cells burst a powerful wave of energy.
This energy shrouded his body until the thunder energy kept destroying his body. It wasn't letting his undead bloodline repair his body in an instant. The moment his blood turned blue thunder pool into red, the shrouding energy disappeared, but not completely.
